Deep percolation, unsaturated lower boundary
If the soil profile is unsaturated, the bottom of the soil profile can either be assumed to be completely impermeable (“No flow”), or a deep percolation of water out of the profile can be simulated in various ways, as determined by the switch LBoundUnSaturated.
If a unit gradient is assumed (“Unit grad flow”) the vertical water flow (deep percolation) is calculated as:
where kwlow is the hydraulic conductivity in the lowest soil layer. It is thus the flow of water from the lowest layer that is the boundary condition that satisfies Richards’s equation (2.2).
The lower boundary can optionally be set by specifying the pressure head in the lowest soil layer i.e. by determining the state variable. When solving Richard’s equation any excess water in the lowest layer is lost from the profile as deep percolation. There are three ways of giving the pressure head at the lowest layer to the model. Either the pressure head is given as a parameter (“Constant Psi”). To satisfy the requirement of this constant pressure head, not only a deep percolation, but also a capillary rise of water from the soil below the lowest layer can occur. The parameter could instead be interpreted as a maximum value (“Constant Maximum Psi”) resulting in a deep percolation when the maximum pressure head is exceeded, but in no capillary rise of water if case of a low pressure head in the bottom layer. Finally the pressure head can be specified as a dynamic variable by giving the values from a PG-file (“Dynamic Psi”). In this case a deep percolation (downward flow) or a capillary rise (upward flow) take place between the lowest soil layer and the soil below in order to satisfy the pressure head requirement in the lowest compartment.